The present disclosure relates generally to positive-displacement devices that include rotors rotatably disposed in stators. More specifically, the present disclosure relates to rotors for positive-displacement devices.
A progressive cavity pump (PC pump) transfers fluid by means of a sequence of discrete cavities that move through the pump as a rotor is turned within a stator. The transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor within the stator, as well as relatively low levels of shearing applied to the fluid. Consequently, progressive cavity pumps are typically used in fluid metering and pumping of viscous or shear sensitive fluids, particularly in downhole operations for the ultimate recovery of oil and gas. Progressive cavity pumps may also be referred to as PC pumps, progressing cavity pumps, “Moineau” pumps, eccentric screw pumps, or cavity pumps.
A PC pump may be used in reverse as a progressive cavity motor (PC motor) by passing fluid through the cavities between the rotor and stator to power the rotation of the rotor relative to the stator, thereby converting the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling. Progressive cavity motors may also be referred to as positive displacement motors (PD motors), eccentric screw motors, or cavity motors. PD motors, or simply mud motors, are used in the directional drilling of oil and gas wells.
Progressive cavity devices (e.g., progressive cavity pumps and motors) include a stator having a helical internal bore and a helical rotor rotatably disposed within the stator bore. Conventional stators often comprise a radially outer tubular housing and a radially inner component disposed within the housing. The inner component has a cylindrical outer surface that is bonded to the cylindrical inner surface of the housing and a helical inner surface that defines the helical bore of the stator. Alternatively, the housing may have a helical bore and the inner component may comprise a relatively thin, uniform thickness coating on the helical inner surface of the housing. In either case, the inner component is typically made of an elastomeric material and is disposed within the stator housing, and thus, may also be referred to as an elastomeric stator liner or insert. The elastomeric stator insert provides a surface having some resilience to facilitate the interference fit between the stator and the rotor. Conventional rotors often comprise a steel tube or rod having a helical-shaped outer surface, which may be chrome-plated or coated for wear and corrosion resistance. The helical internal bore defines lobes on the inner surface of the stator and the helical-shaped outer surface of the rotor defines at least one lobe on the outer surface of the rotor. In general, the rotor may have one or more lobes. To satisfy the fundamental gear tooth law, the stator will have one more lobe than the rotor.
When the rotor and stator are assembled, the rotor and stator lobes intermesh to form a series of cavities. More specifically, an interference fit between the helical outer surface of the rotor and the helical inner surface of the stator results in a plurality of circumferentially spaced hollow cavities in which fluid can travel. During rotation of the rotor, these hollow cavities advance from one end of the stator towards the other end of the stator. Each cavity is sealed from adjacent cavities by seals formed along contact lines between the rotor and the stator. For example, during downhole drilling operations, drilling fluid or mud is pumped through the PD motor as the sealed cavities progressively opening and closing to accommodate the circulating mud. Pressure differentials across adjacent cavities exert forces on the rotor that causes the rotor to rotate within the stator. The centerline of the rotor is typically offset from the center of the stator so that the rotor rotates within the stator on an eccentric orbit. The amount of torque generated by the power section depends on the cavity volume and pressure differential.
In directional drilling, the PC motor is usually positioned at the bottom of a drill string, with the downhole end of the rotor connected to the drill bit via a driveshaft and a shaft concentrically disposed in a bearing assembly and coaxially aligned with the drill bit. To transmit torque from the eccentric rotor to the concentric drill bit, a flexible driveshaft or an articulated driveshaft with universal joints is used to connect the rotor to the shaft of the bearing assembly.
The rotor applies loads to the stator as it rotates therein. The loads come, at least in part, from the work required to rotate the rotor mass within the stator. The loads also come from out-of-balance forces generated as the rotor mass rotates at speed on an eccentric orbit, as well as from other radial forces generated by the rotor mass. The loads can also come from operational circumstances, such as when drilling a curved or deviated section of a borehole. In particular, when drilling a curve, the stator is often bent while the rotor is rotating within the stator. The stator will in turn try to bend the rotor, and the forces from this attempt will be imparted on the stator profile. In general, the higher the loads exerted on the stator by the rotor, the shorter the useful life of the stator.